This invention relates to a tubular ceramic capacitor, and more particularly to a tubular ceramic capacitor which uses a dumet wire for a lead terminal leading out an internal electrode of the same.
A conventional tubular ceramic capacitor, as shown in FIG. 1, has an internal electrode 12 and an external electrode 13 plated with nickel or copper and formed at the inner and outer surfaces of a tubular ceramic dielectric 11 except for both open end faces thereof, a lead terminal 14 which is inserted into a bore of dielectric 11. An internal electrode 12 is formed on the dielectric 11 and is conductively fixed to the lead terminal 14 by solder 15. A metallic cap terminal 16 is fitted onto dielectric 11 in conductive contact with external electrode 13 and provides a lead wire 17 at the outer surface of the closed end of terminal 16.
As shown in FIG. 2, this construction requires a large amount of solder 15 for filling a space between the lead terminal 14 and the dielectric 11 because the former has an outer diameter relatively small with respect to the diameter of the bore of dielectric 11.
Since the linear expansion coefficient of solder is greater than the dielectric 11, a problem is raised in that the internal electrode 12 is peeled off from dielectric 11 when solder 15 expands or contracts. Thus, the adhesion strength of the plated internal electrode 12 is not sufficiently great to resist the larger expansive or contraction force of solder 15, and is therefore apt to peel off.
The simplest and most reliable solution to the above problem is to reduce the amount of solder 15 in the bore of dielectric 11 to fix lead terminal 14 to internal electrode 12.
Therefore, a tubular ceramic capacitor has recently been proposed in which the portion of the lead terminal to be inserted into the bore has a larger diameter than the conventional one, the insertion portion being of length throughout the inner bore, thereby reducing the amount of solder 15. The lead terminal 14 used in such capacitor is made of a usual copper or iron wire. The capacitor having the larger diameter lead terminal 14 can reduce the amount of solder 15, thereby preventing the peeling-off of the internal electrode due to expansion or contraction of the solder 15.
The lead terminal 14, however, when using usual iron or copper wire of a larger diameter, becomes larger in its expansion or contraction due to a larger linear expansion coefficient of the wire itself, which acts on internal electrode 12 to lead to the peeling-off thereof.
Another method has been tried for preventing the peeling-off, in which a silver-coated electrode is attached onto the internal electrode to thereby increase the adhesive strength of the plated internal electrode 12. Such method is defective in that the capacitor is difficult to manufacture, very poor in productivity, and expensive to produce.